The term fabric grain refers to the way threads are arranged in a piece of cloth.
The vertical threads, called warp threads, are stretched on the loom during weaving. They become the fabric's lengthwise grain.
Horizontal threads, called weft threads, are woven back and forth, perpendicular to the warp threads. They make up the fabric's crosswise grain.
The lengthwise grain and crosswise grain are both considered straight grain.
Selvages are the bound edges that run along the outer edges of the lengthwise grain. They are formed when the weft threads turn to change direction as the weaving progresses.
True bias is defined as the direction at a 45 degree angle to the straight grains, but in quilting most folks refer to any cut that doesn't run along a straight grain as a bias cut.
Fabrics that are cut with edges parallel to either the crosswise or lengthwise straight grain are less likely to stretch out of shape than pieces with edges cut along the stretchy bias.
The above information is commonly known. And rules on how to cut have been based on it. It is good just to know. But it is not in anyway a complete approach. Like nothing is unless you work with it. Knowing cloth is simply that. Touching it, tugging on it, taking it apart, weaving it, mending it, using it. Working WITH it.
When working with triangles there is always a bias cut on at least one edge.
Rules are simply a culmination of experience within a certain context. I am thinking that when they are accompanied with a bit of story... how it happened, how it went, what was expected, needed...they make so much more sense, leaving room for considering one's own story. They become softer, like clay. Textbooks should be story books.
Have a good weekend, I'll be back Monday.